Method for forming an ESD protection circuit

ABSTRACT

An ESD protection circuit is formed at the input/output interface contact of an integrated circuit to protect the integrated circuit from damage caused by an ESD event. The ESD protection circuit has a polysilicon bounded SCR connected between a signal input/output interface contact of the integrated circuit and a power supply connection of the integrated circuit and a biasing circuit. The biasing circuit is connected to the polysilicon bounded SCR to bias the polysilicon bounded SCR to turn on more rapidly during the ESD event. The biasing circuit is formed by at least one polysilicon bounded diode and a first resistance. Other embodiments of the biasing circuit include a resistor/capacitor biasing circuit and a second diode triggering biasing circuit.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to electronic circuits, and inparticular to silicon controlled rectifier (SCR) structures forelectrostatic discharge protection (ESD). More particularly thisinvention relates to polycrystalline silicon (polysilicon) bounded SCRstructures and to electronic protection circuits employingpolycrystalline silicon bounded SCR's.

2. Description of Related Art

In ESD protection circuits, the series resistance of the active devicesaffects the performance of the devices. Higher resistance at the voltagelevels of an ESD event may lead to a voltage drop across the activedevices that may destroy the device. FIG. 1 shows an ESD protectiondiode structure of the prior art. In this example, shallow trenches areetched within the region that will become the N-well 10 and filled withan insulating material to form the shallow trench isolation (STI) 15 areformed on the substrate. A semiconductor material that is lightly dopedwith a p-type impurity is formed on the substrate to construct theP-well 5. Within the P-well 5, a lightly doped n-type impurity isdiffused into the P-well 5 to form the N-well 10. Between two of the STIregions 15 a P-type material is diffused into the N-well until a heavilydoped P⁺ region 20 is formed. Similarly, between two other STI regions15 an N-type material is diffused into the N-well until a heavily dopedN⁺ region 25 is formed. An insulative layer 40 is formed on the surfaceof the substrate and opening 32 and 37 are created over the P⁺ region 20and the N⁺ region 25. Silicides 30 and 35 are respectively formed on thesurfaces of the P⁺ region 20 and N⁺ region 25 to create the necessarycontacts to external circuitry. In the case of the ESD protection diodesshown, the contacts will be to the signal input/output interfaceconnection pads and the power supply voltage source connection pads.

According to U.S. Pat. No. 5,629,544 (Voldman, et al.—544), diode seriesresistance is largely determined by the dimensions of the diodefeatures, the resistivity of N-well 10 in which diode is located, thedistance current flows in N-well 10 and the depth of the current path,and by the resistance of contacts 30 and 35 to the p+ and n+ diffusions20 and 25. Thus, a wider diode with a lower well resistivity, a shortercurrent path, and silicided films and contacts provide a lower diodeseries resistance. In the case of the diode as shown, the depth of thecurrent path is determined by the depth of the STI regions 15. Further,it is known in the art that the width of the STI regions 15 have certainachievable minimums that cause the series resistance to be larger thandesired.

Voldman, et al.—544 and “Semiconductor Process and StructuralOptimization of Shallow Trench Isolation-Defined And Polysilicon-BoundSource/Drain Diodes For ESD Networks,” Voldman, et al., ProceedingsElectrical Overstress/Electrostatic Discharge Symposium, October 1998,pp: 151-160 discusses polysilicon-bounded diode. Refer to FIG. 2 formore discussion of the structure of a polysilicon bounded diode. Thestructure of the polysilicon bounded diode is constructed in a P-typewell 5 that has been created with a substrate has been lightly dopedwith a p-type impurity. Within the P-well 5 a lightly doped n-typeimpurity is diffused into the P-well 5 to form the N-well 10. Aninsulative layer 40 is formed on the surface of the substrate. A gatestack is formed with a gate oxide layer 60 and a polysilicon layer 65.Spacers 70 are added to the sides of the gate oxide layer 60 and thepolysilicon layer 65. A P-type material is diffused into the N-welluntil a heavily doped P⁺ region 75 is formed and an N-type material isdiffused into the N-well until a heavily doped N⁺ region 80 is formed oneach side of the gate stack. Openings 77 and 82 are created over the P⁺region 75 and the N⁺ region 80. Silicides 90 and 95 are respectivelyformed on the surfaces of the P⁺ region 75 and N⁺ region 80 to createthe necessary contacts to external circuitry. As described above, thecontacts will be to the signal input/output interface connection padsand the power supply voltage source connection pads.

In the polysilicon bounded diode as shown, the gate stack maybeconstructed with smaller dimensions than those permitted in the diodeconstructed with the STI 15 of FIG. 1. This permits the seriesresistance of the diode to be lower to and thus improves the operationof the diode during an ESD event.

Use of silicon controlled rectifiers (SCR) as ESD protection devices arewell known in the art. Referring to FIG. 3, the P-well 100 isconstructed of a semiconductor material that is lightly doped with ap-type impurity is diffused into a substrate. Within the P-well 100 alightly doped n-type impurity is diffused into the P-well 100 to formthe N-well 105. Shallow trenches are then etched within the region ofthe N-well 105 and filled with an insulating material to form theshallow trench isolation (STI) 110. Between two of the STI regions 110 aP-type material is diffused into the N-well until a heavily doped P⁺regions 125 and 135 are formed. Similarly, between two other STI regions110 an N-type material is diffused into the N-well until the heavilydoped N⁺ regions 120 and 130 are formed. An insulative layer 140 isformed on the surface of the substrate and openings 127 and 137 arecreated over the P⁺ regions 125 and 135 and openings 122 and 132 arecreated over the N⁺ regions 120 and 130. Silicides 145, 150, 155, and160 are formed on the surfaces of the P⁺ regions 125 and 135 and N⁺regions 120 and 130 to create the necessary contacts to externalcircuitry. The contacts will be to the signal input/output interfaceconnection pads and the power supply voltage source connection pads.

The SCR is formed of the P⁺ regions 125, the N-well 105, the P-well 105and the N⁺ regions 130. The anode of the SCR being the P⁺ regions 125and the cathode N⁺ regions 130. As structured, a positive voltage of anESD event applied to the anode will cause the SCR to be activated oncethe snapback voltage is reached. In general the snapback voltage asshown is greater than 50V and may not cause damage to connectedintegrated circuits. However, as the feature sizes of integratedcircuits have become smaller, the voltages at which damage may occur isbecoming smaller and the SCR needs to be triggered at lower voltagesthat are greater than the operating voltages of the integrated circuits.

“Electrostatic Discharge (ESD) Protection in Silicon-On-Insulator (SOI)CMOS Technology with Aluminum and Copper Interconnects in AdvancedMicroprocessor Semiconductor Chips,” Voldman, et al., ProceedingsElectrical Overstress/Electrostatic Discharge Symposium, 1999, pp:105-115, discusses the electrostatic discharge (ESD) robustness ofsilicon-on-insulator (SOI) high-pin-count high-performance semiconductorchips. The ESD results demonstrate that sufficient ESD protection levelsare achievable in SOI microprocessors using lateral ESD SOIpolysilicon-bound gated diodes.

“An ESD Protection Scheme for Deep Sub-Micron ULSI Circuits,” Sharma, etal. Digest of Technical Papers—1995 Symposium on VLSI Technology, 1995,pp: 85-86, describes a scheme for on-chip protection of sub-micron ULSIcircuits against ESD stress using low voltage zener-triggered SCR, and azener-triggered thin gate oxide MOSFET.

U.S. Pat. No. 6,610,262 (Peng, et al.) describes an ESD semiconductorprotection with reduced input capacitance.

U.S. Pat. No. 6,605,493 (Yu) teaches about an SCR ESD protection deviceused with shallow trench isolation structures. The inventionincorporates polysilicon gates bridging SCR diode junction elements andalso bridging between SCR elements and neighboring STI structures. Thepresence of the strategically located polysilicon gates effectivelycounters the detrimental effects of non-planar STI “pull down” regionsas well as compensating for the interaction of suicide structures andthe effective junction depth of diode elements bounded by STI elements.Connecting the gates to appropriate voltage sources such as the SCRanode input voltage and the SCR cathode voltage, typically ground,reduces normal operation leakage of the ESD protection device.

U.S. Pat. No. 6,580,184 (Song) illustrates an ESD protection circuithaving a silicon-controlled rectifier structure. A switch circuit isconnected between a ground voltage terminal and a well region that is abase of the PNP transistor. The switch circuit is formed of pluraldiode-coupled MOS transistors, so that a trigger voltage of the SCR isdetermined by threshold voltages of the MOS transistors.

U.S. Pat. No. 6,534,834 (Ashton, et al.) teaches about a snapback devicethat functions as a semiconductor protection circuit to prevent damageto integrated circuits resulting from events such as electrostaticdischarge. The snapback device includes a polysilicon film overlappingthe active area.

U.S. Pat. No. 5,453,384 (Chatterjee) describes a silicon controlledrectifier structure that is provided for electrostatic dischargeprotection. A polysilicon gate layer is formed over a gate insulatorregion and is electrically coupled to the input pad of an integratedcircuit.

U.S. Pat. No. 5,159,518 (Roy) details an input protection circuit thatprotects MOS semiconductor circuits from electrostatic dischargevoltages and from developing circuit latchup. The input protectioncircuit includes a low resistance input resistor, and two complementarytrue gated diodes.

U.S. Patent Application 2003/0016479 (Song) describes an ESD protectioncircuit having silicon-controlled rectifier structure that includes aPNP transistor and an NPN transistor. A switch circuit is connectedbetween a ground voltage terminal and a well region that is a base ofthe PNP transistor. The switch circuit is formed of plural diode-coupledMOS transistors, so that a trigger voltage of the SCR is determined bythreshold voltages of the MOS transistors.

SUMMARY OF THE INVENTION

An object of this invention is to provide an ESD protection circuit thatbecomes activated at a voltage sufficient to protect integrated circuitsconnected to the protection circuit.

Another object of this invention is to provide an ESD protection circuitwith a polysilicon bounded SCR that conducts of applied energy resultingfrom an ESD event to an input/output interface connection pad.

Still another object of this invention is to provide a bias triggeringcircuit for an SCR that causes the SCR to turn on at a lower voltage inorder to conduct the energy of an ESD event.

A further object of this invention is to provide a diode bias triggeringcircuit for an SCR that causes the SCR to turn on at a lower voltage inorder to conduct the energy of an ESD event.

One more object of this invention is to provide a resistor/capacitortriggering circuit for an SCR that causes the SCR to turn on at a lowervoltage to conduct the energy of an ESD event.

To accomplish at least one of these objects, an ESD protection circuitis formed at the input/output interface contact of an integrated circuitto protect the integrated circuit from damage caused by an ESD event.The ESD protection circuit has a polysilicon bounded SCR and a biasingcircuit. The polysilicon bounded SCR is connected between a signalinput/output interface contact of the integrated circuit and a powersupply connection of the integrated circuit. The biasing circuit isconnected to the polysilicon bounded SCR to bias the polysilicon boundedSCR to turn on more rapidly during the ESD event.

The polysilicon bounded SCR includes a first well region lightly dopedwith impurities of a first conductivity type formed on the substrate andconnected to the power supply connection and a second well region formedwithin the first well region and lightly doped with impurities of asecond conductivity type. A first diffusion region is formed within thesecond well by heavily doping the region with the impurities of thefirst conductivity type. The first diffusion region is connected to thesignal input/output interface contact. A second diffusion region isformed within the first well region at a second distance from the firstdiffusion region by heavily doping the region with impurities of thesecond conductivity type. The second diffusion region is connected tothe power supply connection. A first heavily doped polycrystalline layeris formed at the surface of the substrate and placed between the firstand second diffusion regions and astride a junction of the first wellregion and the second well region to form a bounding component toprevent silicide formation at junctions of the first diffusion regionand the second well region, the first well region and the second regionand the second diffusion region and the first well region duringfabrication of the SCR.

The SCR being the junctions of the first diffusion region and the secondwell region, the junction of the first and second well regions, and thejunction of the first well region and the second diffusion region. Theanode of the SCR is the first well region and the cathode of the SCR isthe second diffusion region.

The biasing circuit is formed of at least one polysilicon bounded diodeformed on the substrate and connected between the signal input/outputinterface contact and an anode connection of the polysilicon bounded SCRto increase a holding voltage for the polysilicon bounded SCR when thepolysilicon bounded SCR is turned on.

The polysilicon bounded diode is formed from the first diffusion regionand the second well region and has a second heavily doped polysiliconlayer formed at the surface of the substrate and placed adjacent to thefirst diffusion region and astride a junction of the second well regionand first diffusion region to form a bounding component to preventsilicide formation at the junction of the first diffusion region and thesecond well region during fabrication of the polysilicon bounded diode.The junction of the first diffusion region and the second well regionforms the polysilicon bounded diode.

The biasing circuit has a first resistance formed by material from thesecond well from a first gate of the polysilicon bounded SCR to a thirddiffusion region formed within the second well, heavily doped with theimpurities of the second conductivity type, and connected to the powersupply connection to provide a low resistance path to the second wellfrom the power supply connection. The biasing circuit further has asecond resistance formed material of the second well from the first gateto the first diffusion region.

A second embodiment of the biasing circuit includes a first resistorconnected from the signal input/output interface contact to the firstgate of the polysilicon bounded SCR and a first capacitor connected fromthe first gate of the polysilicon bounded SCR to the power supplyconnection. When an ESD event occurs, a top plate of the capacitorconnected to the gate of the polysilicon bounded SCR is a virtual groundand the polysilicon bounded SCR is activated.

A third embodiment of the biasing circuit has a plurality of seriallyconnected diodes. The first diode of the plurality of serially connecteddiodes is connected to the signal input/output interface contact and thelast diode of the plurality of the serially connected diodes isconnected to a second gate of the polysilicon bounded SCR. The biasingcircuit further has a second resistor connected from the second gate andthe last diode of the plurality of serially connected diodes to thepower supply connection. When an ESD event occurs, a current flowsthrough the plurality of serially connected diodes and the secondresistor, which triggers the polysilicon bounded SCR to turn on.

A fourth embodiment of the biasing circuit: includes aresistor/capacitor biasing circuit connected from the first gate of thepolysilicon bounded SCR and a diode triggering biasing circuit connectedfrom the second gate of the polysilicon bounded SCR. Theresistor/capacitor biasing circuit has a first resistor connected fromthe signal input/output interface to the first gate of the polysiliconbounded SCR and a first capacitor connected from the first gate of thepolysilicon bounded SCR to the power supply connection. The diodetriggering biasing circuit has a plurality of serially connected diodes.The first diode of the plurality of serially connected diodes isconnected to the signal input/output interface contact and the lastdiode of the plurality of serially connected diodes is connected to asecond gate of the polysilicon bounded SCR. The diode triggering biasingcircuit has a second resistor connected from the second gate and thelast diode of the plurality of serially connected diodes to the powersupply connection.

When an ESD event occurs, the top plate of the capacitor connected tothe gate of the polysilicon bounded SCR is a virtual ground and thepolysilicon bounded SCR is activated. Simultaneously, a current flowsthrough the plurality of serially connected diodes and the secondresistor to trigger the polysilicon bounded SCR to turn on.

The heavily doped polycrystalline layer of the polysilicon bounded SCRand the polysilicon bounded diode permits a series resistance of thepolysilicon bounded SCR and the polysilicon bound diode to be smallerfor a more efficient operation. The heavily doped polycrystalline layeris connected to bias the heavily doped polysilicon layer such thatsalicide shorting is prevented the first and second diffusion regionsand preventing of accidental formation of an inversion region heavilydoped polycrystalline layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional view of a substrate illustrating an ESDprotection diode structure of the prior art.

FIG. 2 is a cross sectional view of a substrate illustrating apolysilicon bounded ESD protection diode structure of the prior art.

FIG. 3 is a cross sectional view of a substrate illustrating an ESDprotection SCR structure of the prior art.

FIG. 4 a is a cross sectional view of a substrate illustrating an ESDprotection polysilicon bounded SCR structure of this invention.

FIG. 4 b is a top plan view of a substrate illustrating an ESDprotection polysilicon bounded SCR structure of this invention.

FIG. 4 c is a plot of current through an ESD protection polysiliconbounded SCR structure of this invention versus the voltage across theESD protection polysilicon bounded SCR structure of this invention.

FIG. 5 is a schematic of a diode triggered ESD protection circuit ofthis invention incorporating ESD protection polysilicon bounded SCRstructure.

FIG. 6 is a schematic of a resistor/capacitor triggered ESD protectioncircuit of this invention incorporating ESD protection polysiliconbounded SCR structure.

FIG. 7 is a schematic of a second diode triggered ESD protection circuitof this invention incorporating ESD protection polysilicon bounded SCRstructure.

FIG. 8 is a schematic of a resistor/capacitor triggered and diodetriggered ESD protection circuit of this invention incorporating ESDprotection polysilicon bounded SCR structure.

FIG. 9 is a schematic of a third diode triggered ESD protection circuitof this invention incorporating an ESD protection polysilicon boundedSCR structure.

FIG. 10 is a schematic of a fourth diode triggered ESD protectioncircuit of this invention incorporating an ESD protection polysiliconbounded SCR structure.

DETAILED DESCRIPTION OF THE INVENTION

The polysilicon bounded SCR of this invention as shown in FIGS. 4 a and4 b includes a P-well 200 lightly doped with p-type impurities formed onthe substrate and connected to the power supply connection 280 throughthe P⁺ diffusion 225. An N-well region 205 is formed within the P-well200 and lightly doped with N-type impurities and connected through theN⁺ diffusion region 220 to the power supply connection 280. A P⁺diffusion region 210 is formed within the N-well 205 by heavily dopingthe N-well 205 with the P-type impurities. The P⁺ diffusion region 210is connected to the signal input/output interface contact 275. An N⁺diffusion region 215 is formed within the P-well 200 at a seconddistance from the N⁺ diffusion region 210 by heavily doping the P-well200 with N-type impurities. The N⁺ diffusion 215 is also connected tothe power supply connection 280.

An insulative material such as a silicon dioxide is formed on thesurface of the substrate between the P⁺ diffusion region 210 and the N⁺diffusion region 215 and astride a junction of the P-well 200 to form agate oxide 230. A polysilicon layer is then formed on the gate oxide 230to form the gate structure 240. The gate structure 240 forms a boundingcomponent to prevent silicide formation at junctions of the P⁺ diffusionregion 210 and the N-well region 205, the P-well 200 and the N-wellregion 205 and the N⁺ diffusion region 215 and the P-well 200 duringfabrication of the SCR.

As is known in the art, an SCR is regarded as a PNP transistor Q1connected serially with an NPN transistor Q2. Thus, the collector of thePNP transistor Q1 is the P-well 200, the base is the N-well 205, and theemitter is the P⁺ diffusion region 210. The collector of the NPNtransistor Q2 is the N-well 205, the base is the P-well 200, and theemitter is the N⁺ diffusion region 215, with the junctions being theboundaries between the P⁺ diffusion region 210 and the N-well region205, the P-well 200 and the N-well region 205 and the N⁺ diffusionregion 215 and the P-well 200.

A N⁺ diffusion region 220 is formed within the N-well 205 by heavilydoping the N-well 205 with the N-type impurities and the P⁺ diffusionregion 225 is formed within the P-well 200 by heavily doping the P-well200 with the P-type impurities. An insulation layer 270 is formed on thesurface of the substrate to protect the surface. Openings 227, 217, 212,and 222 are made in the insulation layer 270 to respectively provideaccess to the P⁺ diffusion region 225, N⁺ diffusion region 215, P⁺diffusion region 210, and N⁺ diffusion region 220. A silicide contact265, 255, 250, and 260 is formed respectively on the surface of each ofthe P⁺ diffusion region 225, N⁺ diffusion region 215, P⁺ diffusionregion 210, and N⁺ diffusion region 220. The silicide contacts 255 and250 are restricted or bounded by the polysilicon gate structure 240.

The diode D₁ is formed as the junction of the P⁺ diffusion region 210and the N-well 205. The gate oxide 235 is formed between and slightlyoverlaps the P⁺ diffusion region 210 and the N⁺ diffusion region 220. Apolysilicon layer is formed on the gate oxide 235 to form the gatestructure 245. The gate structure 245 provides bounding for the suicidecontacts 250 and 260. The polysilicon bounding gate structures 240 and245 permit the P⁺ diffusion region 210 and N⁺ diffusion region 215 to beplaced relatively close by avoiding the necessity for a larger shallowtrench isolation, thus minimizing the serial resistance of the diode D₁and the SCR formed by the transistors Q1 and Q2.

The gate structures 240 and 245 are connected to the power supplyconnection 280 to prevent salicide shorting between the silicidecontacts 265 and 250 and silicide contacts 250 and 260 and preventing ofaccidental formation of an inversion region under said first and seconddiffusion regions. The silicide contacts 265, 255, and 260 provide lowresistivity connections for the P⁺ diffusion region 225, N⁺ diffusionregion 215, and N⁺ diffusion region 220 to the power supply connection280. The silicide contact provides a low resistivity connection for theP⁺ diffusion region 210 to the signal input/output interface pad 275.

Refer now additionally to FIG. 5 for a discussion of the circuitstructure of the polysilicon bounded SCR having a diode triggering ofthis invention. As described above, the diode D₁ is formed as thejunction of the P⁺ diffusion region 210 and the N-well 205. Thepolysilicon bounded SCR is formed of the P-well 200, the N-well 205, theP⁺ diffusion region 210, the N-well 205, the P-well 200, and the N⁺diffusion region 215. The resistor R_(1-SUB) is the bulk resistance ofthe N-well 205 to the P⁺ diffusion region 210. The resistor R_(2-SUB) isthe bulk resistance of the N-well 205 to the N⁺ diffusion region 220.And the resistor R_(3-SUB) is the bulk resistance of the P-well 200 tothe P⁺ diffusion region 225.

Upon application of the voltage of an ESD event to the input/outputinterface pad 275, the diode D₁ begins to conduct. The current throughthe resistance R_(2-SUB) develops sufficient voltage to turn on thetransistor Q1, which in turn provides a current through the resistorR_(3-SUB). This develops a voltage sufficient to turn on the transistorQ2, thus completely activating the SCR.

Referring to FIG. 4 c, as the voltage applied between the anode (P⁺diffusion region 210) and the cathode (N⁺ diffusion region 215) of thePolysilicon bounded SCR increases the current rises slowly 290 until thebiasing of the diode D₁ and the resistor R_(2-SUB) turns on the SCR atthe snapback point 292. At the current and voltage level 294 the SCRfundamentally acts as a resistor, with the resistance determined by theinternal resistance of the SCR. The internal resistance is thendetermined by the dimensions of the SCR and the proximity of the P⁺diffusion region 210 and the N⁺ diffusion region 215. The polysilicongate structures 240 and 245 allow current to flow laterally between theP⁺ diffusion region 210 and the N⁺ diffusion region 215 more efficientlywith a lower resistance to prevent ohmic heating of the device.

A second embodiment of the ESD protection circuit, as shown in FIG. 6 isconnected between the signal input/output interface pad 275 and thepower supply connection pad 280. The SCR is formed as described in FIGS.4 a and 4 b. The diodes D₁ and D₂ are optional diodes placed in serieswith the SCR between the signal input/output interface pad 275 and theSCR. These diodes are structured as the diode D₁ of the FIG. 4 a andincrease the holding voltage of the SCR when it is turned on. The baseof the transistor Q1 and the collector of the transistor Q2 is theN-well 205 of FIG. 4 a and will be referred to as the first gate of theSCR. The base of the transistor Q2 and the collector of the transistorQ1 is the P-well 200 of FIG. 4 a and is referred to as the second gateof the SCR. The resistor R₁ is connected between the signal input/outputinterface pad 275 and the first gate. The capacitor C₁ is connected fromthe first gate to the power supply connection pad 280. In this examplethe power supply connection pad 280 is the ground reference point forthe integrated circuit.

The resistor R_(P-WELL) is the bulk resistance of the P-well 200 of FIG.4 a and is connected from the second gate of the SCR and the powersupply connection pad 280. The resistor R₁ is constructed using anyknown technique such as a highly doped diffusion region. The capacitanceis constructed using any known technique such as employing a gate tobulk capacitance of a MOSFET as the capacitor C₁.

When an ESD event 285 occurs, the voltage at the signal input/outputinterface pad 285 increases dramatically. The top plate of the capacitorC₁ at the first gate is at a virtual ground, thus causing the transistorQ1 to turn on, which causes the current through the resistor R_(P-WELL)to increase and turn on the transistor Q2. The SCR then transfers theenergy to the power supply connection pad 280.

A third embodiment of the ESD protection circuit, as shown in FIG. 7 isconnected between the signal input/output interface pad 275 and thepower supply connection pad 280. The SCR is formed as described in FIGS.4 a and 4 b. As described above, the base of the transistor Q1 and thecollector of the transistor Q2 is the N-well 205 of FIG. 4 a and isreferred to as the first gate of the SCR. The base of the transistor Q2and the collector of the transistor Q1 are the P-well 200 of FIG. 4 aand is referred to as the second gate of the SCR.

The diodes D₁, D₂, and D₃ are serially connected from cathode to anodeand are structured as the diode D₁ of the FIG. 4 a. The anode of thefirst diode D₁ is connected to the signal input/output interface pad 275and the cathode of the last diode D₃ is connected to the second gate ofthe SCR. The resistor R₂ is connected to the second gate of the SCR andthe cathode of the last diode D₃. It should be noted that while thisembodiment is implemented with the three diodes D₁, D₂, and D₃, theremay be any number of diodes connected serially. The number beingdetermined by the operational voltages of the integrated circuitsconnected to the signal input/output interface pad 275.

The resistor R_(P-WELL) is the bulk resistance of the P-well 200 of FIG.4 a and is connected from the second gate of the SCR and the powersupply connection pad 280. The resistors R₁ and R2 are constructed usingany known technique such as a highly doped diffusion region.

When an ESD event 285 occurs, the voltage at the signal input/outputinterface pad 285 increases dramatically. The diodes D₁, D₂, and D₃begin to conduct and a voltage is developed across the resistors R2 andR_(P-WELL). The transistor Q2 turns on causing current to flow throughthe resistor R₁. The voltage developed across the resistor R₁ turns onthe transistor Q2. The SCR is thus activated to conduct the energy ofthe ESD event from the integrated circuits connected to the signalinput/output interface pad 275 to the power supply connection pad 280.

A fourth embodiment of the ESD protection circuit, as shown in FIG. 8 isconnected between the signal input/output interface pad 275 and thepower supply connection pad 280. This embodiment incorporates thetriggering bias circuits of the second and third embodiments. Further,the optional diodes D₁ and D₂ of the second embodiment are included asthe diodes D₄ and D₆ and placed in series with the SCR between thesignal input/output interface pad 275 and the SCR. These diodes arestructured as the diode D₁ of the FIG. 4 a and increase the holdingvoltage of the SCR when it is turned on.

The SCR is formed as described in FIGS. 4 a and 4 b. The base of thetransistor Q1 and the collector of the transistor Q2 is the N-well 205of FIG. 4 a and will be referred to as the first gate of the SCR. Thebase of the transistor Q2 and the collector of the transistor Q1 are theP-well 200 of FIG. 4 a and are referred to as the second gate of theSCR.

The resistor/capacitor triggering circuit is formed by the resistor R1and capacitor C1. The resistor R₁ is connected between the signalinput/output interface pad 275 and the first gate and the capacitor C₁is connected from the first gate to the power supply connection pad 280.In this example, the power supply connection pad 280 is the groundreference point for the integrated circuit.

As described above, the resistor R₁ is constructed using any knowntechnique such as a highly doped diffusion region. The capacitance isconstructed using any known technique such as employing a gate to bulkcapacitance of a MOSFET as the capacitor C₁.

The diode triggering circuit includes the serially connected diodes D₁,D₂, and D₃ and the resistor R₂. The diodes D₁, D₂, and D₃ are seriallyconnected cathode to anode and are structured as the diode D₁ of theFIG. 4 a. The anode of the first diode D₁ is connected to the signalinput/output interface pad 275 and the cathode of the last diode D₃ isconnected to the second gate of the SCR. The resistor R₂ is connected tothe second gate of the SCR and the cathode of the last diode D₃. Asnoted above, that while this embodiment is implemented with the threediodes D₁, D₂, and D₃, there may be any number of diodes connectedserially. The number is determined by the operational voltages of theintegrated circuits connected to the signal input/output interface pad275.

The resistor R_(P-WELL) is the bulk resistance of the P-well 200 of FIG.4 a and is connected from the second gate of the SCR and the powersupply connection pad 280. The resistor R₂ is constructed using anyknown technique such as a highly doped diffusion region.

When an ESD event 285 occurs, the voltage at the signal input/outputinterface pad 285 increases dramatically. The diodes D₁, D₂, and D₃begin to conduct and a voltage is developed across the resistors R₁ andR_(P-WELL). The transistor Q2 turns on. Simultaneously, the top plate ofthe capacitor C₁ at the first gate is at a virtual ground, thus causingthe transistor Q1 to turn on, thus activating the SCR. The SCR thentransfers the energy to the power supply connection pad 280.

A fifth embodiment of the ESD protection circuit, as shown in FIG. 9 isconnected between the signal input/output interface pad 275 and thepower supply connection pad 280. In this embodiment, the triggering biascircuit is a resistor R and capacitor C that are formed of a first metaloxide semiconductor (MOS) transistor M₁ biased to act as the resistor Rand a second MOS transistor M₅ connected to form the capacitor C. TheMOS transistor M₂ connected to bias the MOS transistor M₁ to an oncondition to act as the resistor R. The junction connection between theresistor R and capacitor C is connected to an input terminal of a firstinverter I₁ of a group of serially connected inverters I₁, I₂, and I₃.In this embodiment the preferred implementation of the group of seriallyconnected inverters I₁, I₂, and I₃ is shown as three inverters however,the number of inverters maybe adjusted according to the requirements ofthe design.

The SCR is formed as described in FIGS. 4 a and 4 b. The base of thetransistor Q1 and the collector of the transistor Q2 is the N-well 205of FIG. 4 a and will be referred to as the first gate of the SCR. Thebase of the transistor Q2 and the collector of the transistor Q1 are theP-well 200 of FIG. 4 a and are referred to as the second gate of theSCR.

Each of the group of serially connected inverters I₁, I₂, and I₃ isformed as shown for the inverter I₁. The inverter I₁ is formed of thePMOS transistor M₃ serially connected drain to drain with the NMOStransistor M₄. The gates of the PMOS transistor M₃ and the NMOStransistor M₄ are connected to be the input of the inverter I₁. Thedrains of the PMOS transistor M₃ and the NMOS transistor M₄ being theoutput of the inverter I₁. The source of the NMOS transistor M₄ isconnected to the power supply connection pad 280 and the source of thePMOS transistor M₃ is connected to the diode D₂.

The output of the inverter I₂ that is in phase with the input of thegroup of serially connected inverters I₁, I₂, and I₃ is connected to thefirst gate of the SCR. The output of the inverter I₃ that is out ofphase with the input of the group of serially connected inverters I₁,I₂, and I₃ is connected to the second gate of the SCR.

When an ESD event 285 occurs, the voltage at the signal input/outputinterface pad 285 increases dramatically. The top plate of the capacitorC₁ at the first gate is at a virtual ground, thus activating the groupof serially connected inverters I₁, I₂, and I₃. This causes thetransistors Q1 and Q2 to turn on, thus activating the SCR. The SCR thentransfers the energy to the power supply connection pad 280. The groupof serially connected inverters I₁, I₂, and I₃ provide a sharptransition and a clearly defined window when the SCR is turned on.

The diode D₂ is connected in series with the inverter I₁ to provideprotection against accidental triggering of the ESD protection circuitduring normal operation. The diode D₁ is placed in series with the SCRto increase the holding voltage of the ESD protection circuit. The diodeD₁ may optionally be a group of serially connected diodes to adjust theholding voltage.

A sixth embodiment of the ESD protection circuit, as shown in FIG. 10 isconnected between the signal input/output interface pad 275 and thepower supply connection pad 280. In this embodiment, the triggering biascircuit is a resistor R and capacitor C are formed of a first metaloxide semiconductor (MOS) transistor M₁ biased to act as the resistor Rand a second MOS transistor M₄ connected to form the capacitor C. TheMOS transistor M₂ connected to bias the MOS transistor M₁ to an oncondition to act as the resistor R.

The SCR is formed as described in FIGS. 4 a and 4 b. The base of thetransistor Q1 and the collector of the transistor Q2 is the N-well 205of FIG. 4 a and will be referred to as the first gate of the SCR. Thebase of the transistor Q2 and the collector of the transistor Q1 are theP-well 200 of FIG. 4 a and are referred to as the second gate of theSCR.

The junction connection between the resistor R and capacitor C isconnected to an input terminal of a first inverter I₁ and to the gatesof the PMOS transistor M₇, and the NMOS transistor M₈ within theinverter I₃. The output of the inverter I₁ is connected to the gate ofthe NMOS transistor M₄.

The inverter I₂ is constructed of the PMOS transistor M₃ and the NMOStransistor M₄ having their drains connected together. The source of theNMOS transistor M₄ is connected to the power supply connection pad 280.The source of the PMOS transistors M₃ is connected to the anode of thediode D₂ and the cathode of the diode is connected to the signalinput/output interface pad 275. The output of the inverter I₂ at thejunction of the drains of the PMOS transistor M₃ and the NMOS transistorM₄ is connected to the first gate of the SCR and the gate of the PMOStransistor M₆.

The third inverter I₃ is constructed of the serially connected PMOStransistors M₆ and M₇ and the NMOS transistor M₈. The junctionconnection between the resistor R and the capacitor C is connected tothe gates of the PMOS transistors M₇ and the NMOS transistor M₈, Theoutput of the third inverter I₃ at the junction of the PMOS transistorsM₇ and the NMOS transistor M₈ is connected to the second gate of the SCRand provide a weak feedback to the gate of the PMOS transistor M₃.

When an ESD event 285 occurs, the voltage at the signal input/outputinterface pad 275 increases dramatically. The top plate of the capacitorC₁ at the first gate is at a virtual ground, thus activating the groupof serially connected inverters I₁, I₂, and I₃. This causes thetransistors Q1 and Q2 to turn on, thus activating the SCR. The SCR thentransfers the energy to the power supply connection pad 280. The weakfeedback at the pullup of the PMOS transistor M₃ provides a sharptransition and a clearly defined window when the SCR is turned on.

The diode D₂ is connected in series with the inverter I₁ to provideprotection against accidental triggering of the ESD protection circuitduring normal operation. The diode D₁ is placed in series with the SCRto increase the holding voltage of the ESD protection circuit. The diodeD₁ may optionally be a group of serially connected diodes to adjust theholding voltage.

The ESD protection circuit of this invention, as shown in the sixembodiments, is preferably a polysilicon bounded SCR of this invention.The polysilicon bounded SCR of this invention provides a more compactdevice with a lower internal resistance. However, the ESD protectioncircuit of this invention, as shown in the four embodiments may have ashallow trench isolation bounded SCR as shown in FIG. 3. Further thediodes D₁ and D₂ of FIG. 6, diodes D₁, D₂ and D₃ of FIG. 7, and diodesD₁, D₂, D₃, D₄, and D₅ of FIG. 8 may be the shallow trench isolationbounded diodes as shown in FIG. 1. As is known in the art, the shallowtrench isolation does not allow the small feature size achievable withthe polysilicon bounded SCR or polysilicon bounded diodes. Further thedepth of the shallow trench isolation forces the currents to travelfarther through the bulk of the devices, thus increasing the seriesresistance of the devices and thereby the heating during an ESD event.

While this invention has been particularly shown and described withreference to the preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade without departing from the spirit and scope of the invention.

1. A method for forming an ESD protection circuit formed at aninput/output interface of an integrated circuit formed on a substrate toprotect said integrated circuit from damage caused by an ESD event, saidESD protection circuit comprising: forming a polycrystalline siliconbounded SCR on a surface of said substrate by the steps of: forming afirst well region lightly doped with impurities of a first conductivitytype on a surface of said substrate, forming a second well region withinsaid first well region by lightly doping said second well region withimpurities of a second conductivity type, said impurities of the secondconductivity type having a polarity opposite from the impurities of thefirst conductivity type, forming a first diffusion region within saidsecond well by heavily doping said first diffusion region with theimpurities of the first conductivity type, forming a second diffusionregion within said first well region at a second distance from the firstdiffusion region by heavily doping said second diffusion region withimpurities of the second conductivity type, and forming a heavily dopedpolycrystalline layer at the surface of said substrate and between thefirst and second diffusion regions and astride a junction of the firstwell region and the second well region to form a bounding component toprevent suicide formation at junctions of the first diffusion region andthe second well region, the first well region and the second region, andthe second diffusion region and the first well region during fabricationof said silicon controlled rectifier; connecting said polycrystallinesilicon bounded SCR between a signal input/output interface of saidintegrated circuit and a power supply connection of said integratedcircuit; forming a biasing circuit; and connecting said biasing circuitto said polycrystalline silicon bounded SCR to bias said polycrystallinesilicon bounded SCR to turn on more rapidly during said ESD event. 2.The method of claim 1 further comprising the steps of: forming at leastone diode on said substrate; and connecting said diode between saidsignal input/output interface and an anode connection of saidpolycrystalline silicon bounded SCR to increase a holding voltage forsaid polycrystalline silicon bounded SCR when said polycrystallinesilicon bounded SCR is turned on.
 3. The method of claim 1 wherein saidbiasing circuit: comprises the steps of: forming a plurality of seriallyconnected diodes; connecting a first diode of said plurality of seriallyconnected diodes to the signal input/output interface; connecting a lastdiode of said plurality of serially connected diodes to a second gate ofsaid polycrystalline silicon bounded SCR; and forming a second resistorfrom the second gate and the last diode of the plurality of seriallyconnected diodes to the power supply connection; wherein an ESD eventcauses a current to flow through said plurality of serially connecteddiodes and said second resistor to trigger the polycrystalline siliconbounded SCR to turn on.
 4. The method of claim 1 wherein said heavilydoped polycrystalline layer of the polycrystalline silicon bounded SCRpermits a series resistance of said polycrystalline silicon bounded SCRto be smaller for a more efficient operation.
 5. The method of claim 1wherein forming said biasing circuit: comprises the steps of: forming apolycrystalline silicon bounded diode said polycrystalline siliconbounded diode by the steps of: forming the first diffusion region,forming the second well region, and forming a second heavily dopedpolycrystalline layer at the surface of said substrate and placedadjacent to the first diffusion region and astride a junction of thesecond well region and first diffusion region to form a boundingcomponent to prevent silicide formation at said junction of the firstdiffusion region and the second well region during fabrication of saidpolycrystalline silicon bounded diode; wherein said junction of thefirst diffusion region and the second well region forms saidpolycrystalline bounded diode; and connecting said polycrystallinesilicon bounded diode to the signal input/output interface, forming athird diffusion region within said second well by heavily doping saidthird diffusion region with the impurities of the second conductivitytype, forming a first resistance of material of the second well from afirst gate of said polycrystalline silicon bounded SCR to said thirddiffusion region, and connecting said third diffusion region to saidpower supply connection to provide a low resistance path to said secondwell from said power supply connection.
 6. The method of claim 5 whereinforming said biasing circuit further comprises the steps of: forming asecond resistance of the material of the second well from said firstgate to said first diffusion region.
 7. The method of claim 1 furthercomprising the step of connecting the heavily doped polycrystallinelayer to bias said heavily doped polycrystalline silicon layer such thatsilicide shorting is prevented said first and second diffusion regionsand preventing of accidental formation of an inversion region under saidheavily doped polycrystalline layer.
 8. The method of claim 7 furthercomprising the step of linking the first diffusion region to a voltagesource which provides a relatively large voltage during said ESD eventwhich when said relatively large voltage exceeds said snapback voltage,said polycrystalline silicon bounded SCR conducts.
 9. The method ofclaim 7 further comprising the step of connecting said heavily dopedpolycrystalline silicon layer to the second diffusion region.
 10. Themethod of claim 9 further comprising the step of linking the first wellregion, the second well region, said highly doped polycrystallinesilicon layer, and the second diffusion to a return of the voltagesource.
 11. A method for forming an ESD protection circuit formed at aninput/output interface of an integrated circuit formed on a substrate toprotect said integrated circuit from damage caused by an ESD event, saidESD protection circuit comprising: forming a shallow trench isolationbounded SCR on a surface of said substrate; counecting said shallowtrench isolation bounded SCR between a signal input/output interface ofsaid integrated circuit and a power supply counection of said integratedcircuit; forming a biasing circuit; counecting said biasing circuit tosaid shallow trench isolation bounded SCR to bias said shallow trenchisolation bounded SCR to turn on more rapidly during said ESD event;forming at least one diode on said substrate; and counecting said diodebetween said signal input/output interface and an anode connection ofsaid shallow trench isolation bounded SCR to increase a holding voltagefor said shallow trench isolation bounded SCR when said shallow trenchisolation bounded SCR is turned on.